1. Field of the Invention
The invention relates to imaging systems and more particularly to protecting image capturing devices.
2. Description of Related Art
In general, an integrated circuit device or chip is made up of thousands to millions of individual devices coupled together in an integrated way through conductive metal lines. The interconnection of individual devices requires multiple layers of metal lines that are insulated from one another by dielectric material, such as for example silicon dioxide (SiO.sub.2) or silicon nitride (Si.sub.3 N.sub.4). Located on the periphery of the integrated circuit chip are bond pads. To activate the circuitry within the chip, it is necessary to supply voltage and input signals to the bond pads. These voltage signals are supplied to the bond pads through a package to which the integrated circuit device is affixed. Once affixed to a package, individual bond wires are used to electrically couple each bond pad of a chip to a corresponding pad on a package substrate. Each corresponding pad on a package substrate is then individually coupled to an external pin. The packaged integrated circuit device may then be placed within a socket in order to electrically couple the external pins to drivers that supply the necessary voltage and input signals to activate the integrated circuit chip. The bond pads also provide signals from the chip to other components of the system in which the chip is operating.
Digital imaging systems, such as for example digital cameras, utilize integrated circuit devices or chips as image capturing devices. An image capturing device, such as a camera, uses light to capture an image by a semiconductor-based chip. The chip replaces film in traditional film-based systems. In a camera, an image capturing device is configured, in its simplest form, to capture a monochrome or color image by way of field effect transistors (FETs), such as complementary metal oxide semiconductor (CMOS) devices or charge couple devices (CCDs).
In one example, the image capturing device is a chip made of a number of photodiodes, each photodiode capable of absorbing light. In color applications, each photodiode generally absorbs light through a color filter and represents one color corresponding to the image sensed. FIG. 1 illustrates an image capturing device or chip 100. FIG. 2 shows a schematic, planar side view of chip 100. Image capturing device 100 is fabricated as part of a die or wafer 10 with a plurality of other devices. Once formed, individual image capturing devices are separated from one another typically by a sawing process.
Image capturing device 100 contains a plurality of pixels, such as for example 480 rows by 640 columns. In general, each pixel contains a photosensing structure, such as a photodiode, and other pixel circuitry. The photosensing structure is the region of the pixel that responds to light while the pixel circuitry drives the light signal from the photosensing structure to other process circuitry. In FIGS. 1 and 2, the light capturing components are represented by sensor area 110.
In addition to sensor area 110, image capturing device 100 contains additional logic circuitry 120 that operates the individual pixels and drives signals from the pixels off device 100. In FIGS. 1 and 2, the device circuitry occupies an area of device 100 represented by logic area 120. It is to be appreciated that the device circuitry is not limited to logic area 120. Logic area 120 represents an area around the periphery of device circuitry as opposed to photogenerating structures, like photodiodes. To power device 100 and to drive signals off device 100, device 100 contains bond pads 125. Bond pads 125 are generally arranged on the periphery or top surface of device 100 and surround sensor area 110 and logic area 120. Bond pads 125 are typically located on the extreme periphery of device 100 in bond pad area 140. Bond pads 125 are electrically linked or coupled to device circuitry 122. Logic area 120 and bond pad area 140 collectively define a non-sensor area separate from sensor area 110.
The top surface of image capturing device 100 includes passivation layer 130. Passivation layer 130 is typically silicon nitride (Si.sub.3 N.sub.4) or silicon oxynitride (Si.sub.x O.sub.y N.sub.z) because of these materials resistance to environmental contaminants, particularly moisture. Si.sub.3 N.sub.4 or Si.sub.x O.sub.y N.sub.z passivation layers 130 in suitable thicknesses to serve as a passivation layer, such as for example approximately 8000 angstroms (.ANG.), are also transparent and therefore may overlay sensor area 110 without negative effects on the image capturing capabilities of sensor area 110. One problem associated with Si.sub.3 N.sub.4 or Si.sub.x O.sub.y N.sub.z passivation layers 130 is that the layer is hard and brittle and subject to cracking. The microprocessor and other device applications may maintain their high integrity and use a passivation layer of Si.sub.3 N.sub.4 or Si.sub.x O.sub.y N.sub.z, because these microprocessor structures are typically overlaid with a photodefineable polyimide. The polyimide, among other things, protects the Si.sub.3 N.sub.4 or Si.sub.x O.sub.y N.sub.z passivation layer during high volume assembly processes, most importantly the device pick-and-place process where an individual microprocessor device is selected, for example, by a collet and transferred into a package.
The polyimide used in microprocessor applications is not transparent and therefore will negatively effect imaging capturing devices. Thus, any polyimide that is used to protect a Si.sub.3 N.sub.4 or Si.sub.x O.sub.y N.sub.z passivation layer of an image capturing device during handling must be removed, generally by exposing the image capturing device to a wet etch such as for example with a chemical solvent, before packaging.